In retinal imaging, one seeks to obtain photographs of the detail of the optic nerve and the surrounding retinal material that heretofore has been captured on film.
A constant problem in retinal imaging is the fact that the optic nerve, which is basically where all of the nerve bundles go back to the brain, is very reflective. When one fires a strobe into the eye to illuminate the retina, the optic nerve tends to throw large amounts of light back at the camera, which shows up as a bright blotch at the position where the optic nerve is attached to the retina. The rest of the detail of the retina that one wants to see, especially at the periphery, is very dark. The result of photographing the retina utilizing high-power strobe pulse illumination is a very high-contrast image where the darker regions are drowned out by the high reflectivity of the optic nerve.
More importantly, the optic nerves of the darker-skinned races, including the Negroid and Hispanic races, are in general more highly reflective than white or Caucasian races, making retinal imaging even more difficult. It is noted that people of darker complexion have darker retinas because the pigment of the retina is darker.
When taking retinal images, one has to inject enough light to illuminate the dark areas. However, if one increases the output of the xenon strobe normally used, one simply drowns out the optic nerve detail because of its high reflectivity.
In the past, in order to obtain images of the detail of the optic nerve as well as images of the retina, one had to take numbers of photographs, each with different light outputs or different F stops on the camera. With multiple pictures one needs multiple strobe pulses, with each strobe pulse injecting energy into the eye. This causes pain and is very uncomfortable for the patient. What this means is that a patient may have to endure a number of 100 watt-second pulses discharged into the eye. It is therefore desirable to be able to eliminate the requirement for multiple exposures.
However, the problem is not so much seeing the remainder of the optic nerve but dealing with the high reflections where the optic nerve is attached to the retina that visually resembles a hole in the back of the eye. As will be appreciated, the optic nerve is always on the nasal side of the eye and appears as an offset bright hole. On the other hand, the retina has blood vessels and arteries that stretch out across the eye including smaller capillaries that branch off. In general, the vascular structure forms a circular pattern. At the center of vision, which is called the macula, there are no blood vessels and therefore it is completely devoid of blood vessel structure.
It is noted that in addition to the optic nerve, pathology can be highly reflective as well. High reflections can come from a scar, tumor or growth, the reflections from which will saturate the camera with the introduction of a pulse from the xenon strobe.
As mentioned hereinabove, one technique to eliminate the problems of being able to view the structure of the eye is to have multiple photographs, one to expose the optic nerve and the other to expose the rest of the retina. In order to get the detail of the optic nerve, one could either reduce the output of the flash lamp or stop down the camera so that just this area is properly exposed to be able to see all the detail. However, by cutting down the flash power to be able to observe the optic nerve detail, one has insufficient light to be able to view the remainder of the retina. Note that with too high a strobe output the strobe saturates the camera due to the reflectivity of the optic nerve and all detail is gone.
As will be apparent, by raising the flash lamp power output one would simply see hotspots in the image for which detail is completely lacking.
In the past, in order to be able to view the detail of the retina away from the optic nerve, the typical practice was to slightly increase the flash lamp output which, while causing hotspots near the optic nerve or the pathology, it was possible to discern the detail of the darkened portion of the retina.
There is therefore a necessity for eliminating the requirement for multiple photographs, both from a patient comfort point of view and to be able to view all of the retina in a single image or photograph.
With the advent of digital cameras, usually having CCD sensor arrays of a 1- to 11-megapixel variety, it is possible to get real-time images of the retina. However, the problem of multiple pictures and flash lamp intensity versus optic nerve reflectivity has not as yet been resolved for the above reasons.